I've been home for the holidays, and have been drinking quite a bit of my Mum's homemade kombucha. Kombucha is pretty much a microbiologist's dream drink. Along with beer… But that's a different blog post.
Kombucha is a tea-like drink traditionally consumed in China, Russia, and Germany. At different times throughout history, kombucha has been thought to be magical, increase the immune response, and even cure cancer. So what is in this stuff?! Simply put, it's a fermented, carbonated, slightly sweet black tea. The thing that does the fermenting part, which I think is the coolest part of kombucha, is the "SCOBY": Symbiotic Culture Of Bacteria and Yeast. The SCOBY has been called a few different things throughout history, such as "Tea Fungus", or, my favourite, "Manchurian Mushroom". With every new batch of kombucha produced, a new layer of SCOBY grows (a "baby SCOBY") on top of the "Momma SCOBY". The new baby SCOBYs can be used to make new kombucha batches – i.e. to ferment the next batch of tea.
Most people who drink kombucha have a vague feeling that it's good for them, and have heard that the SCOBY is made up of "good" microbes. In this blog post, we're going to dig in a little deeper.
Let's start with the SCOBY itself. Its name already describes its contents pretty well, but it's not just one type of bacteria and one type of yeast contained within this vibrant microbial community. It is the many different yeasts in kombucha that kick-start the fermentation process. Kombucha is made using black tea and sugar, and the yeast in the culture convert this sugar to ethanol. That's right, ethanol, as in the alcohol in your beer or wine. As for the particular yeast species in SCOBYs, various scientific studies have consistently found inconsistency. Whatever you have brewing in your kombucha cauldron depends highly on where you got your SCOBY. However, there are a few yeasts that are common to most household samples: those from the species, Saccharomyces, Zygosaccaromyces, and Brettanomyces.
The ethanol produced by these yeasts is in turn used as food for the bacteria in the SCOBY. Most SCOBY bacteria are producers of acetic acid and gluconic acid. A genus of bacteria called Acetobacter is the most common in the floating "Manchurian Mushroom". These bacteria are also able to produce cellulose, which is a major component of the SCOBY, and helps it float to the top of your brew to let SCOBY bacteria that are oxygen-dependent breathe. Along with Acetobacter species, Gluconobacter and Lactobacillus have also been identified in many different SCOBYs from around the world. As is suggested by their names, these ones produce gluconic and lactic acids. Lactobacillus in particular is also one of the "good" bacteria to have in your digestive system!
My mum's glorious SCOBY
If you want to know even more about the microbes in your SCOBY, this paper by Greenwalt, Steinkraus, and Ledford includes a nice table with more detail. Here on Microbial Mondays, however, we'll now turn to the liquid part of the kombucha that we drink!
Most scientific studies of kombucha indicate that the healthy effects of drinking it comes from the acetic, lactic, and gluconic acids in the fermented tea. In particular, gluconic acid has some antibacterial properties. It has also been claimed that glucuronic acid (which sounds pretty much the same but is actually rather different than gluconic acid. I was confused, too.) in kombucha can help the liver with detoxification processes. But, glucuronic acid is either present in very small quantities or not at all in kombucha, so I'm not convinced of that. There has also been a lot of talk about kombucha being a cancer cure. However, this is based on studies in Russia that found that villages that made kombucha had lower rates of cancer than villages that made vodka. Vodka is a carcinogen. I'm pretty sure it wasn't so much the kombucha but the lack of really strong alcohol that helped.
There were two health effects that I found most convincing and well-backed by good science. The first was that kombucha seems to have some antibacterial activity against some harmful bacterial species such as Helicobacter pylori, which causes ulcers. Secondly, any teas that contain chemicals called polyphenols can help balance the bacteria in your intestines, by inhibiting the growth of "bad" bacteria like Clostridia and helping the growth of "good" bacteria like Bifidobacteria. This holds true for kombucha.
As for the SCOBY yeast and bacteria, I also haven't found any scientific research about whether many of these microbes are floating around in the tea, or if it helps to drink them. Normally, the bacteria and yeasts are arranged in bands along the cellulose in the floating SCOBY, and I'm really not sure how many detach. However, I do know that Lactobacillus, which is often in SCOBYs, is generally good for you: it's the same "good" bacteria in your yoghurt.
All in all, there have been many health claims about kombucha's powers, but very few have been backed up by good science. As far as I know, there also haven't been any long-term studies of kombucha's health effects (let me know if you know of a good study!). I'm hoping that some science on this will pop up though, as kombucha's popularity spreads with the hipster and yoga movements. For now, I wouldn't get too excited about kombucha being a wonder drug… But it sure tastes good!
With that, Happy New Year, all! And if you don't feel like drinking vodka for a while after New Year's Eve… Try kombucha!
Alex